This invention relates to radio receiver circuitry and, in particular, to circuitry for removing a varying DC distortion component of a received information packet signal.
Wireless (radio) information signals are transmitted in packets with each packet typically containing several portions such as an initial preamble portion, a synchronizing portion and a data portion, the packets having been appropriately modulated for radio transmission. Due to a number of interference sources to which the radio signal becomes subject prior to decoding in the receiver and other factors such as TX frequency drift during transmission, each received packet is prone to have a high amplitude perturbation at the beginning and then a slow drift thereafter until the end of the packet. This varying amplitude constitutes a varying DC distortion component of the received signal and, disadvantageously, causes errors in the conversion and decoding of the demodulated data signal to an output data signal.
To deal with this DC distortion component of the demodulated data signal it is known to use an AC coupling RC network or some other linear filtering circuitry, to remove the initial high amplitude (i.e. baseline) component and thereby fix the DC level of the preamble portion of the packet. However, these known solutions do not address the problem of the slow varying DC distortion which occurs throughout the packet. U.S. Pat. No. 5,629,960 to Dutkiewicz et al provides a method for reducing a DC distortion component (interference) produced during the transmit mode of a duplex packet switched data communications system. This interference in the receiver occurs during the period of the transmit ON/OFF keying on the transmitter and this is a known event. Dutkiewicz et al provide a DC off-set tracking filter, operative on the demodulated received signal after it has been converted to a digital signal and immediately prior to the symbol timing recovery stage, having a bandwidth which is fixed on a timed basis such that the timing of the operation of the fixed bandwidth filter matches the timing of the ON/OFF keying. Disadvantageously, however, this solution is directed only to a fixed (known) source of DC interference.
Accordingly, there is a need for a means of removing multiple contributory sources of varying DC off-set in a received signal in order to restore the signal to a level DC set-off.
In accordance with the invention there is provided DC restoration circuitry for a communications receiver for removing a varying DC component of an information signal comprising a stream of symbols (e.g. radio packets), the receiver comprising symbol recovery circuitry including a decision component is for evaluating symbol acquisition values (e.g. symbol integrals or symbol samples) and producing a decision for each evaluated symbol acquisition value. An adaptive filter produces estimates of input symbol acquisition values which are correlated to the decisions made by the decision component for previously input symbol acquisition values (but which are not correlated to the DC component) and subtracts these estimates from the input symbol acquisition values to produce a feedback error signal which is used to restore the DC level of the stream of symbols. The error signal represents the varying DC component. Further, an amplitude estimator component is preferably provided for estimating the amplitude of the received symbols and producing an amplitude estimate signal which is used by the adaptive filter for scaling the decisions produced therein.
The symbol recovery circuitry may include an integrate and dump component configured for producing the symbol acquisition values for evaluation by the decision component and a symbol timing recovery component, the symbol acquisition values being symbol integrals and the error signal being subtracted from the symbol integrals output from the integrate and dump component.
The adaptive filter may comprise an LMS controller configured for controlling the correlation of the decisions for producing the estimates. Preferably, a correlator module is provided for detecting the presence and ending of a preamble portion of the received information packet and a preamble filter component provides a preamble DC component of the received stream of symbols whereby a preamble ending DC component is provided on detection of the ending of the preamble portion, together with switching means for causing subtraction of the preamble DC component from the received stream of symbols when the presence but not the ending of the preamble is detected and for instead causing subtraction of the preamble ending DC component from the received stream once the ending of the preamble has been detected.
This invention uses an adaptive filter, and a corresponding adaptive (equalization) feedback signal, in an unusual and surprising manner to remove a slow varying (exponential) DC component in a data signal. In a receiver decoder the adaptive filter (e.g. an LMS filter) compares the outputs of an xe2x80x9cintegrate and dumpxe2x80x9d or xe2x80x9csample and holdxe2x80x9d component, for example, to adapted outputs of a decision component in order to produce an error signal which inherently corresponds to the varying DC component of the received symbol stream. This equalizing adaptive error signal is fed back and subtracted from the symbol integral stream to remove the varying DC component therefrom and the DC restored integral stream is fed to the decision component. Although this DC restoration circuitry is advantageously suited for use in receivers for packet data communications systems it could also be used in receivers for synchronous data systems.